An electrical motor produces its rotating mechanical power as the result of forces generated by the interaction between the stationary magnetic field of the stator and the magnetic field developed in the rotor windings as an electrical current passes through these windings. In order to obtain an electrical contact between the rotor and a power source it is sometimes necessary to employ a rotating or sliding electrical contact. The rotating part of the contact is a commutator which is made of copper segments connected to the windings. These segments allow the current to pass through the windings and are situated so the current may be reversed as the rotor rotates. In addition, there are two or more stationary contacts called brushes. These brushes contact the rotating commutator segments, thus allowing conduction of electricity between a stationary power source and the rotating windings.
The brushes are typically made of a carbon and graphite mixture, sometimes with a metal powder such as copper powder added. The brushes are usually manufactured by mixing the carbon and/or graphite and, if desired, the metal powder with a binder. The resulting mixture is formed into a large block and is baked at a high temperature. The block is then cut into plates, and the plates are cut into individual brushes of the desired size.
In selecting a brush and/or a brush material, there are several properties or factors that must be considered. One such property is specific resistance, which is the measure of the brushes ability to carry an electrical current. The specific resistance is a function of the cross-sectional area of the brush and electrical conductivity of the brush material. Ideally, a brush should have a low specific resistance in order to lower the voltage drop across the brush and thereby reduce the power loss.
Another important property is brush hardness. Hardness gives an indication of how the brush will operate when in use. A very hard brush tends to vibrate and slip, while a softer one runs more quietly. The strength of a brush is also important since it determines the ability of the brush to withstand compression, tension and shear.
Another factor which must be considered in evaluating a particular brush or brush material is brush wear and abrasiveness. With continued operation of an electrical motor an insulating deposit is seen to build up on the commutator. Therefore, a certain amount of abrasion by the brushes on the commutater is desirable to remove this build up. However, excessive abrasiveness will cause the brushes to wear much too quickly, thus bringing the motor operation to a halt. Therefore, any brush and/or material selected must show a proper balance between wear and abrasion.
One last factor that must be considered is contact voltage drop. This is a meausre of the electric contact between the brushes and the commutater. Clearly, the better the electrical contact, the more efficiently the motor will run.
The properties outlined above are also important in the design and manufacture of electrical contacts such as pantograph contacts, sliding or breaking electrical contacts, conducting gears and bearings, and electrostatic discharge machinery tools. The above properties are important in selecting the materials used in these devices as well.
It is therefore an object of this invention to provide improved electric motor brushes and other electrical contacts containing graphite.
It is another object of this invention to provide electric motor brushes and other electrical contacts having low specific resistance and a long, useful life.
It is still another object of this invention to provide improved electrical contacts having good wear and current carrying capabilities.
It is still another object of this invention to provide electrical motor brushes and other electrical contacts with a low short circuit current.